I've recently had to learn how to upload data collected on a GPS for work. I've installed JOSM and with some minor difficulties, was able to upload some marks I made last week. The issue I'm having is that JOSM doesn't have a map background for me to see the marks against. I tried using the 'download map data from OSM' button but all I can see are tiles with red borders and Xs. After clicking the download button and waiting a bit, I get this error:

'Failed to open a connection to the remote server 'http://api.openstreetmap.org/api/'. Please check your internet connection.'

I've tried to do some troubleshooting, basically trying to find a manual download links for slippymap, to no avail. How do I get a proper background map to see my marks against?